The damage to a fuel pipeline that sparked last month's aviation crisis is now believed to have happened three years ago.
A digger driver damaged the pipeline supplying aviation gas to Auckland Airport in 2014, the Northland Regional Council confirmed.
However, the identity of the digger driver who damaged the buried pipeline at Ruakaka remains a mystery.
The council released details of its investigation into the massive leak this morning which saw thousands of air passengers on international and domestic flights affected for about a week.
Council officers have spoken to the landowner's wife about the matter in some detail.
Evidence collected showed a digger was on site in the area of the damaged pipeline about three years ago.
This had been confirmed by both the landowner's wife and a neighbour.
The council also used satellite imagery to see if it showed a digger working in the area in the relevant period but regular cloud cover hampered visibility.
To date, no one had been able to confirm who was operating the digger when it struck the pipe but the council was now following a "strong line of inquiry".
It said it was unable to provide more detail to avoid compromising this aspect of the investigation.
The council said the landowner's wife had been co-operative and had also spoken independently to representatives of both NZ Refining and its insurers.
Efforts were now focused on ensuring a methodical, careful and thorough investigation into the cause of the discharge.
The council had until March next year to lay any potential charges relating to the spill.
Results of groundwater sampling near the pipeline rupture showed bore water supplies had not been contaminated by the leaking fuel.
The Northland council was continuing to work with Refining NZ on repairing the site and clarifying necessary resource consents.
